There are known mechanical devices, both manual and automatic, designed to connect or disconnect the wheels from a vehicle's optional drive axles in order to achieve or obtain a significant fuel economy. This occurs when the wheels are released from the referred optional drive axles since part of the power generated by the engine will no longer be wasted in moving the components of the optional drive system that exist between the vehicle's wheels and the transfer case.
The prior art manual mechanical devices are not convenient since the driver has to step out of the vehicle to connect or disconnect the optional drive. The conventional automatic devices have the disadvantage of not allowing for engine compression braking.
As further background for the present invention, free wheeling hub assemblies are designed to drivingly connect wheels to the associated driving wheel axles which are provided with splines and upon which end of the axles the hubs are mounted. It is known that four wheel drive vehicles are meant to operate in conditions under which the maximum traction capability is required. When these conditions arise, the second driving axle, most usually the front one, is engaged by transferring engine power to it through a transfer case.
Under normal 4.times.2 conditions, however, the front power train is disconnected from the transfer case, but continues to turn, "dragged" by the wheels. This results in the additional resistance which increases fuel consumption and tire wear, causing unnecessary wear of the front power train moving parts, and reduced driveability.
These undesirable consequences are eliminated by the use of a hub clutch on each wheel, through which it is possible to disconnect the wheels from the drive shafts when desired. In vehicles equipped with the free wheeling hubs, when driving in 4.times.2, the wheels will be the only moving parts of the front drive line, just like in any conventional 2 wheel drive vehicle.
In the last three decades, numerous free wheeling mechanisms have been devised including manual hubs, automatic roller type hubs, automatic cam type hubs and axle disconnecting mechanisms. Manual hubs connect or disconnect the wheels to the drive shafts through different mechanical means, the most reliable of which consists of a sliding clutch ring, contained in a housing fixed on to the wheel hub, and externally controlled by a manual dial. Although the manual free wheeling hub serves its purpose, and is durable and trouble free, it has the obvious disadvantage of the driver having to get out of the vehicle every time the four wheel drive is required.
In automatic roller type hubs, when torque is applied to the drive shafts, the hubs lock themselves through the wedging of rollers between an inner polyhedric body splined on the shaft and the race of a housing, bolted on the wheel hub. Thus, the axle can drive the wheels but the wheels will not drag the power train. The main inconvenience of this mechanism is that engine compression braking cannot be applied to the front wheels, which, in difficult off-road operation, can be a serious handicap to the control of driver over vehicle.
In automatic cam type hubs the same principle as in the manual hub is applied, only that instead of being manually operated, the sliding clutch ring is displaced by a combination of cams and friction mechanisms when torque is applied to the drive shaft. To disengage the hubs, it is necessary to stop the vehicle and reverse it, for the clutch ring to return to its original position. Although this type does allow for engine compression braking, its reliability appears to be doubtful. In addition to the inconvenience of having to back up the vehicle, this rather complicated construction requires regular servicing, can be noisy in rough off-road situations, such as forward/reverse vehicle rocking, and is subject to failure under extreme low temperatures.
In a disconnecting mechanism or system, instead of featuring a locking device on each wheel, it consists of a split drive shaft on one side of the front axle gear. The shaft remains split when operating in 4.times.2 and is locked together by a remote control sliding sleeve for 4.times.4 operation. This system does not really provide a full free wheeling effect. Even in 4.times.2, the wheels drag the drive shaft on one side, part of the drive shaft on the other, and the differential gears. Only the ring/pinion gear and propeller shaft remain still when in 4.times.2. Although the system provides for the comfort of an in-cab operation the free wheeling effect is only partial since a substantial number of parts is permanently connected to the wheels. The vehicle's axle must also be specially designed and engineered to accommodate the shaft split mechanism.
Applicant attemted to solve the inconveniences of the prior art devices by developing a totally automatic mechanical device incorporating engine braking as is disclosed in his Brazilian Patent No. PI-8102668, filed Apr. 30, 1981. In this automatic mechanical device, the wheels are connected automatically when the optional drive is actuated by means of the transfer case lever. Disconnection is accomplished by disengaging the optional drive, also through the transfer case lever. However, in order to disconnect the wheels completely, it is necessary to move the vehicle in the opposite direction to that of its previous movement.
Even though the automatic mechanical device described in the above patent did present some advantages when compared to other existing devices at that time, the applicant continued his studies and research towards obtaining a more practical solution to the problem of the prior art since his automatic mechanical device still presented the inconvenience of having to move the vehicle in a direction contrary to that of its previous movement in order to completely disengage the wheels.